1. Field of the invention
The present invention relates to an image position detecting device and, more particularly, to a device which includes a light receiving element and a signal processing circuit for detecting the position of the image formed on the light receiving element.
2. Description of the Prior Art
There have been proposed various image position detecting devices and one of which is disclosed in U.S. Pat. No. 3,723,003 to Vockenhuber et al. issued Mar. 27, 1973 (corresponding to German Patent Offenlegungsschrift No. 1 956 014 laid open to public on Oct. 1, 1970). According to U.S. Pat. No. 3,723,003, the light receiving element is formed by an array of a plurality of light receiving units, such as photodiodes, and a circuit receives output from each of the light receiving units for detecting which one of the light receiving units is receiving the image. Since each light receiving unit must have a certain size, and since a number of light receiving units are provided to obtain a certain level of accuracy, the light receiving element comes relatively large in size.
An improved image position detecting device similar to that described above is disclosed in U.S. Pat. No. 4,251,144 to Matsuda et al. issued Feb. 17, 1981 (corresponding to German Patent Offenlegungsschrift No. 29 09 090 laid open to public on Sept. 20, 1979). According to U.S. Pat. No. 4,251,144, the circuit for detecting the position of the image permits the detection of an image which is formed on a single unit of an array of light receiving units, or on two or more units in a bridged manner so as to improve the accuracy of the detection. The circuit, however, is relatively complicated and requires a number of constructing parts.
Another type of image position detecting device is disclosed in Tokkosho (Japanese Patent Publication) No. 50-29330. According to Tokkosho No. 50-29330, the light receiving element is defined by a rectangular photoconductive plate, such as CdSe, having an electrode belt deposited on one long side of the plate and an ohmic resistance belt deposited on the opposite long side of the plate. Each of the belts has a terminal at the end of the belt for the external connection. An optical arrangement is provided in front of the light receiving element so as to form a line image which crosses on the element such that the line image extends from the electrode belt and crosses the photoconductive plate perpendicularly to the resistance belt. When the line image is sufficiently bright, a section on the photoconductive plate where the line image is impinged changes its resistance to approximately zero and if the remaining sections on the photoconductive plate are sufficiently dark, a current path is established over the line image between the electrode belt and the resistance belt, and accordingly, establishes a closed circuit between the terminals through a part of the electrode belt, the photoconductive plate along the line image and a part of resistance belt. The length of the resistance belt occupied in the closed circuit corresponds to the position of the line image, and thus, permits a detection of the position of the line image.
Although the image position detecting device according to Tokkosho No. 50-29330 has a simpler structure than the first mentioned device, it has the following disadvantages:
(a) The photoconductive plate has a poor sensitivity and its response speed is considerably long. Therefore, it can only detect a bright object moving relatively slow. In the case where the image position detecting device has a source of light for emitting light beams towards the object and receives the reflected light, such a light source must produce intense light beams that can form a bright image on the light receiving element. The employment of such a high power light source results in a bulky size of the device; and
(b) Since the image is presented in a form of line, and since it must cross the electrode belt and resistance belt, it is necessary to adjust the location of line image in its lengthwise direction. In other words, if the line image fails to cross either the electrode belt or resistance belt, the image position detecting device will not provide a correct position of the image on the light receiving element.